The electric power system in the United States generates three-phase alternating current (AC) electric power. Each power phase is 120 degrees out of phase, plus or minus, with the other two power phases. The voltage of any phase oscillates sinusoidally between positive voltage and negative voltage. Three-phase electric power generation, transmission, and distribution provides an acceptable compromise between the efficiency, expense, and complexity of power system equipment.
It is more efficient to transmit electric power at high voltage levels than at low voltage levels. Electric power may be generated as three-phase AC power at moderate voltage levels in the 12 kilovolt (kV) to 25 kV range. The voltage level may be stepped up to the 110 kV to 1000 kV range using a transformer for transmission over long transmission lines, hence minimizing transmission line power loss. The transmission line voltage may be stepped down, using a transformer at a substation, to the 12 kV to 35 kV range for local distribution. The local distribution voltage level may be further stepped down through one or more transformer stages to provide 120 volt AC power to residential and commercial structures. Special accommodations may be made for electric power consumers at manufacturing plants. In some contexts, the electric power system may be abstractly categorized into electric power generation, electric power transmission over extended distances, and electric power distribution to electric power consumers.